The present invention relates generally to communication systems and more particularly relates to a method of avoiding collisions among multiple transmitters in an asynchronous communications system.
Currently, high frequency radio frequency (RF) communications is becoming more and more prevalent in the world. Products touting wireless RF communication links are becoming increasingly popular among consumers. Today, there are an increasing number of new products, including redesigned existing ones, being designed that incorporate wireless RF links.
Many of today""s applications of RF wireless technology require low cost solutions. Products are designed with extraneous components eliminated from the final product. In numerous applications, it is necessary to communicate data from one point to another but in a unidirectional rather than bidirectional manner. For example, a device may be able to transmit but not receive, i.e., it cannot receive acknowledgements of its transmissions. Notwithstanding this, it is desirable to still be able to reliably communicate between the transmitter and the receiver. In addition, many applications include a plurality of transmitters communicating asynchronously with each other to a single receiver.
To achieve reliable communications, techniques have been developed which increase the odds of successful communications. For example, one technique is to repeat the same message a particular number of times. Even if interference prevents the reception of one or more of the messages, it is not likely that every transmission of the repeated message will be prevented from being received.
The problem, however, is compounded in certain types of multiple transmitter communication systems. For example, a problem exists in frequency hopping spread spectrum and other types of systems wherein all the transmitters begin transmitting bursts of data packets asynchronously with respect to each other. It is entirely possible that two transmitters may begin transmissions at nearly the same time.
If the transmitters in the system utilize different pseudo random hop sequences, than the problem is less severe. If the transmitters in the system utilize the same hop sequence, and same hop time, however, then it is possible that two transmitters will begin transmission at nearly the exact same time and interfere with each other throughout all transmissions (on all frequencies). The occurrence of this could cause complete loss of the transmitted data. Thus, the overall link quality of the communication system can be enhanced by a transmission arrangement that minimizes the chance of collision of transmissions from asynchronous multiple transmitters.
The present invention comprises a method of avoiding collisions among a plurality of transmitters that communicate asynchronously in relation to each other with a single receiver. The transmission arrangement of the present invention utilizes a randomization process to minimize the probability of repeated collisions among transmitted signals from different non synchronized transmitters, thus achieving improved overall system performance.
The method of the present invention has applications to many types of communications systems, in particular, frequency hopping spread spectrum communications systems. The invention has utility in any application where a message is communicated by repeatedly transmitting the message a multiple number of times in order to increase the reliability of reception.
To aid in illustrating the principles of the present invention, the following description is presented within the context of a frequency hopping spread spectrum communication system. The scope of present invention, however, is not limited to such an application. One skilled in the art can apply the principles of the present invention to numerous other types of communications systems as well.
The invention comprises a plurality of transmitters wherein each transmitter transmits data in a burst of data packets, each data packet sent during a separate time slot within the burst. Further, each time slot comprises two or more sub time slots. During each time slot, one of the sub time slots is randomly chosen and the data packet is transmitted during that particular time. A larger number of sub time slots used results in a lower probability of collision and complete transmission loss. By randomizing the timing of the transmission of the data packets among the plurality of sub time slots, the chances of collisions in all time slots throughout the burst is greatly reduced.
There is provided in accordance with the present invention, in a communications system consisting of a plurality of transmitters and at least one receiver, each the transmitter including means for initiating and transmitting a message to the at least one receiver, a method of transmitting the message, the method comprising the steps of transmitting the message a predetermined number of times N, each message transmission occurring during a time slot thus yielding a burst consisting of N time slots, subdividing each time slot into a plurality of M sub time slots, randomly transmitting the message during one of the M sub time slots while not transmitting during the remaining Mxe2x88x921 time slots and wherein N is a positive integer and M is equal to or greater than 2. The method further comprises the step of appending a gap time to the initial portion of each the time slot before the plurality of sub time slots.
There is also provided in accordance with the present invention, in a frequency hopping spread spectrum communications system consisting of a plurality of transmitters and at least one receiver, each the transmitter including means for initiating and transmitting a message to the at least one receiver, each transmitter utilizing the same pseudo random hop sequence of N frequencies, a method of avoiding collisions between transmission signals, the method comprising the steps of transmitting the message N times, each message transmission occurring during a time slot wherein each time slot is assigned a different frequency, subdividing each time slot into a plurality of M sub time slots, randomly transmitting the message during one of the M sub time slots while not transmitting during the remaining Mxe2x88x921 time slots and wherein N is a positive integer and M is equal to or greater than 2.